1. Field of the Invention
The present invention relates to delayed coker unit operations. Specifically, the present invention relates to isolation valves used in the delayed coker unit operation.
2. Background
Petroleum refining operations in which crude oil is processed frequently produce residual oils that have very little value. The value of residual oils can be increased when processed in a delayed coker unit (“DCU”). Residual oil, when processed in a delayed coker is heated in a furnace to a temperature sufficient to cause destructive distillation in which a substantial portion of the residual oil is converted, or “cracked” to usable hydrocarbon products and the remainder yields a residual petroleum by-product called “coke,” a material composed mostly of carbon. A large vessel hereafter called a “coke drum” is provided at the furnace outlet to allow sufficient residence time for the hydrocarbons to complete destructive distillation reaction.
The production of coke is a batch process. Each delayed coker unit module vessel usually contains more than one coking drums, and a fractionation tower. In delayed coking, the feed material is typical residuum from vacuum distillation towers and frequently includes other heavy oils. The feed is heated as it is sent to one of the coke drums. The feed arrives at a coke drum with a temperature ranging from 870 to 910° F. Typical drum overhead pressure ranges from 15 to 35 PSIG. The oil flows directly from the furnace to a coke drum. Coker feedstock is deposited as a hot liquid slurry in a coke drum. Under these conditions, cracking proceeds and lighter fractions produced flow out of the top of the coke drum and are sent to a fractionation tower where they are separated into vaporous and liquid products. A solid, residuum called coke is also produced and remains within the drum. When a coke drum is filled, residual oil from the furnace is diverted to another coke drum. When a coke drum is filled to the desired capacity, and after feedstock is diverted to another drum, steam is typically introduced into the drum to strip hydrocarbon vapors off of the solid material. The liquid mass remaining in the coke drum cools and is quenched as a part of the process. Solid coke formed as the drum cools must be removed from the drum so that the drum can be reused. While coke is being cooled in one or more drums and while the cooled coke is being extracted from one or more drums, other drums are employed to receive the continuous production of coke feedstock as a part of the delayed coker process. The use of multiple coke drums enables the refinery to operate the fired heater and fractionating tower continuously. Drum switching frequency ranges from ten to 24 hours.
Because most delayed coker unit operations consist of several modules, each typically alternating between two coke drums in the coking/decoking sequence, each delayed coker unit must repeatedly be isolated from and reconnected to the flow of hot residuum, and must be repeatedly connected to a port that allows lighter fractions produced during cracking to escape from the vessel back to the fractionator.
Isolation valves presently used in the coking operations are typically placed in tandem, and operators utilize a double block and bleed method for ensuring the isolation of the flow of matter through a given line. The double block and bleed method is an expensive and redundant means for ensuring line isolation.
Some valves utilized in the delayed coker unit operation must be maintained for long periods in a partially open position. For example the back warming valve isolation valve in traditional decoker units is maintained in a partially open position for extended periods. As a consequence, debris or other matter may build up on the internal components of the valves which then become inoperable.